This invention relates to acoustic devices in which two or more regions of differing acoustic wave velocity are desired.
Acoustic wave filters, particularly acoustic surface wave filters, appear to have increasing potential for use in commercial applications such as commercial television circuits. For example, a filter for such an application typically employs a region of a dielectric crystal in which a substantially periodic pattern of perturbations of velocity are imposed, typically by metal strips on the surface of the crystal or by ion beam etching depressions in the surface of the crystal in the desired substantially periodic pattern. A bandwidth broader than that obtainable from a uniform spacing is obtained in the filter by providing a selected, reproducible variation in the spacing of the metal stripes or other surface loading stripes.
In the unrelated optical device art, it has recently been shown that the diffusion of metals or their oxides into lithium tantalate or into lithium niobate changes the optical refractive index of the crystal in the region where the diffusion took place. Such techniques have resulted in a simplification of optical devices which require two or more regions of differing optical index of refraction. Although the same materials, lithium niobate and lithium tantalate, also have elastic properties making them usable in some acoustic devices, initial analysis tended to be discouraging as to the possibility of obtaining analogous device simplification in the acoustic device as in the optical device. Not only was it impossible to predict whether an appreciable effect on acoustic velocity would result from such techniques; but it was also impossible to predict even the sense of the effect. Perhaps most importantly, it was thought that most metals that could be introduced into the bulk of lithium niobate or lithium tantalate might have a decidedly adverse effect on the overall loss of acoustic devices, as compared to devices which use only the prior art surface loading techniques.